1. Field of the Invention
The present invention relates to a control method of an exhaust gas purification system and an exhaust gas purification system for conducting exhaust gas temperature rise control accompanied by operation of an exhaust brake valve and an exhaust throttle valve in order to recover purification capacity of a diesel particulate filter device to purify components in an exhaust gas of an internal combustion engine such as a diesel engine.
2. Description of the Related Art
Regulations on an amount of the particulate matter (PM: particulate matter: Hereinafter referred to as PM) exhausted from a diesel engine as well as NOx, CO, HC and the like have been tightened year by year. A technology has been developed that the PM is collected in a filter called a diesel particulate filter (DPF: Diesel Particulate Filter: Hereinafter referred to as DPF) to thereby reduce the PM amount exhausted to the outside. A continuous regeneration type DPF device carrying a catalyst is among them.
In this continuous regeneration type DPF device, when an exhaust gas temperature is approximately 350° C. or above, the PM collected by the filter is continuously burned and purified, and the filter is self-regenerated. However, if the exhaust temperature is low, for example, if a low exhaust temperature state such as in an idling operation, a low-load/low-speed operation, or the like of an internal combustion engine continues or the like, a temperature of the exhaust gas is low, and the temperature of the catalyst is lowered and the catalyst is not activated. Thus, oxidation reaction is not promoted, and oxidation of PM to regenerate a filter becomes difficult. Therefore, accumulation of PM on the filter continues, and clogging of the filter progresses. Thus, there emerges a problem of exhaust pressure rise caused by the clogging of the filter.
One of methods to solve such a problem is a regeneration control for forcedly burning and removing the collected PM by forcedly raising the temperature of exhaust gas when an amount of clogging of the filter exceeds a predetermined one. In this regeneration control, the exhaust gas temperature rise control is conducted to raise a temperature of the exhaust gas flowing into the filter to the temperature or above at which the PM collected by the filter is burned. By this arrangement, the filter temperature is raised so as to burn and remove the PM and the filter is regenerated.
This kind of exhaust gas temperature rise control includes methods of fuel injection in a cylinder (in-cylinder) such as multiple injection (multiple-stage delayed injection), post injection (after-injection). The multiple injection is a delayed multiple-stage injection in which the fuel is injected into the cylinder in many stages. By this multiple injection, a fuel amount simply burned in the cylinder without generating torque is increased, and the temperature of the exhaust gas exhausted from the cylinder, that is, the temperature of the exhaust gas flowing into an oxidation catalyst device can be raised to a catalyst activation temperature of the oxidation catalyst or above.
The post injection is an auxiliary injection injecting at timing further delayed from the multiple injection injected after main injection in the in-cylinder injection. By this post injection, HC (hydrocarbon) is increased in the exhaust gas exhausted from the cylinder and the HC is oxidized by the oxidation catalyst. By this oxidation, the temperature of the exhaust gas on the downstream of the oxidation catalyst device can be raised.
On the other hand, in view of a measure against oil dilution that an unburned fuel is mixed with engine oil (lubrication oil) by the post injection and dilutes the engine oil, there is a control of forced regeneration during idling when the vehicle is parked and when an operation state is stable. In this control, when the PM is accumulated in a predetermined amount in the filter device, need of regeneration control of the filter device is notified to an operator (driver) by a warning means such as an alarm lamp. When the driver having received the notification stops the vehicle and presses a manual regeneration button, a manual regeneration mode is started and the forced regeneration is carried out.
In this system, the oxidation catalyst device is installed on a front (upstream side) of the filter device and the HC supplied into the exhaust gas by the post injection is oxidized through this oxidation catalyst device. By this oxidation, the temperature of the exhaust gas at an inlet of the filter device is raised so as to carry out the forced regeneration.
In this exhaust gas temperature rise, when the temperature of the exhaust gas is low as in the low-speed/low-load operating state, the multiple injection is carried out in the first place so as to raise the exhaust gas temperature of the oxidation catalyst device to a catalyst activation temperature of the oxidation catalyst or above. After the exhaust gas temperature of the oxidation catalyst device has been raised to the catalyst activation temperature, the post injection is carried out by the fuel injection control while the exhaust gas temperature is kept equal to the catalyst activation temperature or above and HC is supplied to the oxidation catalyst device. This HG is oxidized by the oxidation catalyst and generates heat, and thus the exhaust gas flows into the filter device with the temperature further raised. By this high-temperature exhaust gas, the PM accumulated in the filter device is burned and removed.
At this regeneration, as described in Japanese Patent Application Kokai Publication No. 2005-76604 and Japanese Patent Application Kokai Publication No. 2004-353529, for example, the following forced regeneration is carried out. If the temperature of the exhaust gas flowing into the oxidation catalyst device is equal to the oxidation catalyst activation temperature or below, the temperature is kept by throttling an exhaust throttle valve (exhaust throttle) and the multiple injection is carried out at the same time. By this multiple injection, the temperature of the exhaust gas flowing into the oxidation catalyst device is raised to the oxidation catalyst activation temperature or above. After that, the forced regeneration is carried out by further carrying out the post injection or by carrying out the exhaust throttle, the multiple injection and the post injection at the same time.
In the exhaust temperature rise control using the exhaust throttle valve as above, if the exhaust throttle valve is set to the fully closed side in the regeneration control when a vehicle is parked, an exhaust passage is narrowed. Thus, resistance against the exhaust gas flow is increased, the exhaust pressure is raised, and an engine load (pumping loss) of an engine is increased. The increase of the engine load is one of factors that reduce an engine speed. However, during an idling operation, control is conducted so that an idling engine speed is maintained. Thus, a fuel injection amount into a cylinder is increased, and more amount of heat is generated. Accordingly, an in-cylinder temperature and a temperature of the exhaust gas are raised.
On the other hand, from the viewpoint of measures against noise, slight opening is provided at the exhaust throttle valve even in the fully closed state, and heat escapes from this opening, that is, a gap even at the fully closed time. Thus, if the temperature of the exhaust gas is to be raised to a temperature required for the forced regeneration, it is necessary to further generate heat on the engine side for the escaping heat. As a result, the engine might be overheated.
Then, an exhaust brake valve (exhaust brake) with valve opening (opening area) further smaller than the exhaust throttle valve at fully closed may be used to raise the exhaust gas temperature. However, in this case, if the post injection is carried out with the exhaust brake valve fully closed, the opening area of the exhaust passage becomes too small. Thus, though the exhaust gas temperature is rapidly raised, the in-cylinder temperature is also raised too much. As a result, when the in-cylinder temperature is low, ignition is not realized and the fuel by the post injection flowing out to the exhaust passage side is ignited and burned in the cylinder, which causes abnormal combustion inside the cylinder. Since the post injection is carried out considerably after a TDC (top dead center), burned gas flows into the exhaust. Thus, the exhaust gas temperature becomes abnormally high, which damages the oxidation catalyst and DPF. Therefore, there is a problem that the temperature of the exhaust gas can not be raised by the post injection with the exhaust throttle by the exhaust brake valve.
Patent Document 1: Japanese Patent Application Kokai Publication No. 2005 - 76604
Patent Document 2: Japanese Patent Application Kokai Publication No. 2004 - 353529